1. Field of the Invention
The present invention relates to an image forming apparatus having an image forming process unit employing an appropriate method such as electrophotography, electrostatic recording or magnetic recording. More specifically, the present invention relates to an image forming apparatus, which forms a toner image on a surface of a transfer material.
2. Description of the Related Art
Conventionally, an image forming apparatus, such as a copying machine or a printer, employs an electrophotographic method. In the electrophotographic system, a toner image is formed on a transfer material such as a paper by toner particles using an electrostatic force, melted and fixed on the transfer material by heat and pressure applied by a fixing device, and discharged as an output image. In recent years, image forming apparatuses employing the electrophotographic method have improved functionalities such as color imaging function and high-speed processing. Under the circumstances, a color image forming apparatuses employing a tandem system that is excellent in functionality are widely used. A conventional configuration described below also employs the tandem system.
FIG. 7 illustrates a schematic diagram of a color image forming apparatus employing the tandem system. In the tandem system color image forming apparatus, toner image forming units 1a, 1b, 1c and 1d for colors of cyan, magenta, yellow, black are independently arranged. A transfer material S such as a paper and an overhead projector (OHP) sheet is conveyed by an electrostatic conveying belt 7 made from a resin material that is stretched by a driving roller 8 and a driven roller 9, and sequentially passes through the respective toner image forming units 1a, 1b, 1c and 1d. Each time the transfer material S passes through the toner image forming unit, each color toner image is superimposed on the other color toner images on the transfer material S to form a full-color toner image on the transfer material S. The image forming processing will be described in more detail below. Since internal operations of the respective toner image forming units are almost the same, only the operation of the toner image forming unit 1a for cyan will be described on behalf of all colors. In the toner image forming unit 1a, a photosensitive drum 2a that is driven to rotate in an arrowed direction is, first, uniformly charged to a negative potential by a photosensitive drum charger 3a and a latent image corresponding to a cyan image is formed on the surface by a scanning light from a laser exposure optical system 4a. A given amount of negatively charged cyan toner is supplied onto a developing roller 5a, and a developing bias is applied to the developing roller 5a. The developing bias is set to an appropriate value between a potential of a uniformly charged portion and a potential of an exposed latent image portion so that the toner can selectively adhere to the latent image on the photosensitive drum 2a and be developed.
The cyan toner image formed on the photosensitive drum 2a is electrostatically transferred onto the transfer material S by a transfer bias of positive polarity opposite to the charged polarity of the toner applied to a transfer roller 14a. The transfer material S is conveyed at an approximately same speed as the photosensitive drum 2a in a close contact with the electrostatic conveying belt 7.
The toner remaining on the photosensitive drum 2a that has not been transferred is scraped off by a cleaning blade 6a and collected into a waste toner container (not illustrated).
The above-described process is performed by each of the toner image forming units (1b, 1c, and 1d) for yellow, magenta and black. When a full-color unfixed toner image is formed on the transfer material S, the transfer material S is separated from the electrostatic conveying belt 7 and is guided to a fixing device 18.
In the fixing device 18, the transfer material S passes through a pressure contact portion (fixing nip) between a fixing roller and a pressure roller, where the unfixed toner image is melted and fixed on the transfer material S by heat and pressure as an output image of the apparatus.
An attraction roller 12 assists in attracting the transfer material S onto the electrostatic conveying belt 7. A voltage of positive polarity is applied to the attraction roller 12. The transfer material S supplied from a sheet feeder 16 passes between the electrostatic conveying belt 7 and the attraction roller 12. As a consequence, electrostatic polarization between the transfer material S and the electrostatic conveying belt 7 is promoted, thus attracting the transfer material S onto the electrostatic conveying belt 7.
The transfer material S retained by the electrostatic conveying belt 7 sequentially passes through image forming stations for respective colors and each time the transfer material S undergoes a transfer process, the transfer material S receives many charges on its surface. As a result, the transfer material S after the transfer process is in a strongly charged state and separating discharge occurs when the transfer material S is separated from the electrostatic conveying belt 7 which may disturb an unfixed toner image on the transfer material S.
To solve the foregoing problems, an electric discharge device is provided at a separating position of the electrostatic conveying belt. The transfer material is separated from the electrostatic conveying belt to discharge the transfer material to an appropriate level as discussed, for example, in Japanese Patent Application Laid-Open No. 2004-004335 and Japanese Patent Application Laid-Open No. 2000-206802.
The electric discharge device disposed at the transfer material separation position of the electrostatic conveying belt includes a discharge electrode and a shield electrode that is a conductive member for assisting electric discharge of the discharge electrode. When a voltage of a threshold value or higher to the discharge electrode is applied, electric discharge is generated between the discharge electrode and the shield electrode to remove excessive charges from the transfer material, thus occurrence of the separating discharge is prevented. However, deterioration of the discharge electrode may be accelerated if the electric discharge device continuously generates electric discharge between the discharge electrode and the shield electrode. Accordingly, an electric discharge operation of the electric discharge device is stopped between paper sheets during image formation or under a condition where an amount of charge of the transfer material is prone to decrease. Stopping the electric discharge device can prevent acceleration of the deterioration of the discharge electrode. The amount of charge of the transfer material is prone to decrease in a high-humidity environment.
However, if the image formation is performed while the operation of the electric discharge device is stopped, the toner may adhere to and soil the electric discharge device. More specifically, negatively charged toner is attracted by mirror image charges of positive polarity induced by the shield electrode or the discharge electrode which has a ground potential in a voltage OFF state and come flying to adhere to the electric discharge device.
While the electric discharge device generates electric discharge between the discharge electrode and the shield electrode, soiling is less likely to occur even though a voltage of positive polarity which is more prone to attract the toner is applied to the discharge electrode. This is because the electric discharge generated between the discharge electrode and the shield electrode enables ion molecules which are much lighter than toner particles swiftly to reach and discharge the toner on the transfer material.
As a measure against soiling of the electric discharge device, a structure can be considered in which a voltage of the same polarity as a charge polarity of the toner is independently applied to each of the discharge electrode and the shield electrode while the operation of the electric discharge device is stopped to prevent the toner from adhering to the electric discharge device. Such a structure is illustrated in FIG. 8.
The structure shown in FIG. 8 includes a discharge electrode 55 and a shield electrode 53 which is a flat-shaped conductive member. A power circuit 56 capable of applying voltages of both positive and negative polarities is connected to the discharge electrode 55. More specifically, the power circuit 56 is connected to a terminal 58 to apply a voltage of negative polarity, and to a terminal 59 to apply a voltage of positive polarity. Further, a power circuit 57 capable of applying a voltage of negative polarity is connected to the shield electrode 53.
During an operation (electric discharge operation) of the electric discharge device, a voltage of +4 kV is applied to the discharge electrode 55 and the shield electrode 53 is set OFF (0 V) to accelerate discharge between the discharge electrode 55 and the shield electrode 53 so that ions are supplied. When the electric discharge operation is not performed (discharge OFF state), a voltage of −1 kV is applied to both of the discharge electrode 55 and the shield electrode 53 to prevent adhesion of the toner.
By applying a voltage of the same polarity as the charge polarity of the toner to both of the discharge electrode 55 and the shield electrode 53 as described above when the electric discharge operation is not performed, soiling of the electric discharge device can be reduced.
The above-mentioned operations may be summarized in the following table 1.
TABLE 1During electricDuring electricdischarge operationdischarge OFFDischarge4 kV−1 kVelectrode(output from powercircuit 56)Shield electrode0 kV−1 kV(output from powercircuit 57)
However, in the structure that soiling of the electric discharge device can be reduced by applying a voltage of the same polarity as the charge polarity of the toner to both of the discharge electrode 55 and the shield electrode 53 when the electric discharge operation is not performed, a relationship of potentials between the power circuit 56 of the discharge electrode 55 and the power circuit 57 of the shield electrode 53 is considered to vary from a state illustrated in Table 1.
This is because separate output switches are provided for the power circuit 56 of the discharge electrode 55 and the power circuit 57 of the shield electrode 53 which causes a little time lag of output values when the electric discharge operation is switched to the discharge OFF state and vice versa.
FIGS. 9A and 9B illustrate a state of voltage switching of a discharge electrode (illustrated by solid line) and a shield electrode (illustrated by a broken line). FIG. 9A illustrates a case where a voltage applied to the discharge electrode is switched earlier than that applied to the shield electrode. FIG. 9B illustrates a case where the voltage applied to the shield electrode is switched earlier than that applied to the discharge electrode.
In a region P2, a potential difference between the discharge electrode and the shield electrode is larger than 4 kV which is a value during an ordinary electric discharge operation. In such a state, a discharge current increases and deterioration of the discharge electrode accelerates. On the other hand, in a region P1, the potential of the discharge electrode is lowered earlier than that of the shield electrode. Periphery of the electric discharge device and a surface of the shield electrode may have a very small amount of soil and toner which is charged with a positive polarity due to positive discharge during the electric discharge operation. Accordingly, the toner and soil receive a force of an electric field in the region P1 to move toward and adhere to the discharge electrode.
Deterioration and soiling of the discharge electrode reduces discharge performance and causes image failure during separation of the transfer material. Moreover, a strong electric field force generated between the toner which has not been discharged and the discharge electrode may cause a white spot phenomenon in which a part of a toner image is drawn toward the discharge electrode and lost.